1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus for imparting assist force to a steering system of an automobile or other vehicle by use of a motor, and more particularly to a motor-driven power steering apparatus having a step-up circuit capable of regulating current supplied from an on-vehicle battery to the motor.
2. Description of the Related Art
Conventionally, there has been used a motor-driven power steering apparatus which assists operation of a steering wheel through utilization of rotational force of a motor. In such a motor-driven power steering apparatus, when a driver rotates the steering wheel to steer the vehicle, the motor imparts to a steering mechanism a steering assist force corresponding to a steering torque that the driver applies to the steering wheel.
Incidentally, the above-described motor-driven power steering apparatus is a system that requires large current in order to produce large torque.
Conventionally, since voltage output from an on-vehicle battery (DC 12 V) is applied directly to a motor-driven power steering apparatus, a motor designed to operate at DC 12V is used therein, with the result that large current is supplied to the motor. Therefore, the size of the motor unavoidably increases, and the capacity (thickness) of wiring lines to be used unavoidably increases.
In order to solve the above problem, there has been proposed a motor-driven power steering apparatus capable of regulating current supplied from an on-vehicle battery to a motor (Japanese Patent Application Laid-Open No. 8-127350).
In the motor-driven power steering apparatus, as shown in FIG. 51, a step-up circuit 300 and a step-up circuit controller 301 are provided in a circuit for supplying current to the motor. The step-up circuit 300 is provided between a point P1 to which battery voltage VPIG (DC 12V) is applied from an on-vehicle battery and a point P2 from which voltage is applied to the motor. The step-up circuit 300 includes capacitors C1 and C2, a coil L, a diode D, and a switching transistor Q1.
The step-up circuit controller 301 outputs a duty-ratio drive signal to the transistor Q1 of the step-up circuit 300 in order to control the transistor Q1 according to the duty-ratio drive signal (duty control). By virtue of this duty control, the transistor Q1 performs switching operation as shown in FIG. 52. As a result, accumulation of energy in the coil L and discharge of energy from the coil L are repeated, and high voltage generated by virtue of discharge appears on the cathode side of the diode D. Note that, in FIG. 52, Txcex1 represents an ON period; T represents a pulse cycle; and xcex1 represents a duty ratio (on duty). These definitions will be used throughout the present specification. When the transistor Q1 is turned on, current flows through the coil L, and when the transistor Q1 is turned off, the current flowing through the coil L is cut off.
When the current flowing through the coil L is cut off, high voltage is generated on the cathode side of the diode D so as to prevent magnetic flux from changing in response to cut off of the current. Through iteration of this operation, high voltage is generated repeatedly on the cathode side of the diode D. The high voltage is smoothed by means of the capacitor C2 and is fed to the point P2 as output voltage VBPIG.
The voltage obtained through step-up effected by the step-up circuit 300 relates to the duty ratio of the duty-ratio drive signal output from the step-up circuit controller 301. When the duty ratio increases, the output voltage VBPIG increases; and when the duty ratio decreases, the output voltage VBPIG decreases.
However, since the conventional step-up circuit 300 uses the diode D as described above, a problem arises when the motor enters a regeneration state. That is, even when the motor enters a regeneration state, current cannot flow from the voltage application point P2 toward the battery B, due to presence of the diode D, so that the output voltage VBPIG increases. Due to this increased voltage, the step-up circuit 300 may be broken. In the example shown in FIG. 51, the capacitor C2 and the diode D, which constitute the step-up circuit 300, may be broken.
In view of the foregoing, an object of the present invention is to provide a motor-driven power steering apparatus in which a step-up circuit is not broken even when a motor enters a regeneration state.
In order to achieve the above object, the present invention provides a motor-driven power steering apparatus for a vehicle, comprising: a steering mechanism for steering the vehicle in accordance with operation of a steering wheel; a motor mechanically coupled to the steering mechanism and generating assisting force to be imparted to the steering mechanism; control signal generation means for generating a motor control signal on the basis of at least steering torque applied to the steering wheel; motor drive means for driving the motor in accordance with the motor control signal; a step-up circuit provided in a current supply circuit extending from a battery to the motor drive means, the step-up circuit including a step-up coil whose one end is connected to the battery for reception of battery voltage, a first switching element connected between the other end of the step-up coil and the ground, a second switching element connected between the other end of the step-up coil and an output terminal, and a capacitor connected between the output terminal and the ground and smoothing output voltage which is produced by means of the step-up coil and appears at the output terminal; and step-up circuit control means for detecting the output voltage and controlling the first and second switching elements on the basis of difference between a target output voltage and the detected output voltage in such a manner that during power generation periods, at least the first switching element is turned on and off in order to increase the output voltage supplied to the motor through step-up operation, and during regeneration periods, at least the second switching element is turned on and off.
In the motor-driven power steering apparatus having the above-described configuration, the step-up circuit does not break even when the motor enters a regeneration state.
Preferably, the first and second switching elements are each constituted by a field effect transistor. In this case, the reliability and efficiency of the motor-driven power steering apparatus can be improved.
Preferably, the step-up circuit control means turns the first and second switching elements on and off alternately on the basis of the difference between the target output voltage and the detected output voltage in such a manner that during power generation periods, the output voltage supplied to the motor is increased through step-up operation, and during regeneration periods, regenerative current output from the motor flows to the battery.
In this case, the output voltage supplied to the motor can be increased through step-up operation during power generation periods, and regenerative current output from the motor can flow to the battery during regeneration periods.
Preferably, the motor-driven power steering apparatus further comprises steering state judgment means for determining, on the basis of the difference between the target output voltage and the detected output voltage, whether the motor is in a power generation state or in a regeneration state, wherein the step-up circuit control means turns at least one of the first and second switching elements on and off on the basis of the state of the motor determined by the steering state judgment means.
This configuration prevents the output voltage from increasing during regeneration periods.
The step-up circuit control means may turn only the first switching element on and off when the steering state judgment means determines that the motor is in a power generation state, and turn the first and second switching elements on and off alternately when the steering state judgment means determines that the motor is in a regeneration state.
In this case, since the first switching element is turned on and off during power generation periods, heat generation and loss during power generation periods can be reduced as compared with heat generation and loss in a diode used in conventional apparatuses.
Alternatively, the step-up circuit control means may turn only the first switching element on and off when the steering state judgment means determines that the motor is in a power generation state, and turn only the second switching element on and off when the steering state judgment means determines that the motor is in a regeneration state.
In this case as well, heat generation and loss during power generation periods can be reduced as compared with that of a diode used in conventional apparatuses. Further, since the second switching element is turned on and off during regeneration periods, heat (loss) produced by current which flows through the second switching element in on periods can be reduced, whereby efficiency can be increased.
Alternatively, the step-up circuit control means may turn the first and second switching elements on and off alternately when the steering state judgment means determines that the motor is in a power generation state, and turn only the second switching element on and off when the steering state judgment means determines that the motor is in a regeneration state.
In this case, during power generation periods, the second switching element is turned on for step-up operation in order to reduce the quantity of heat (loss) generated by current which flows through the second switching element in on periods, as compared with that generated by current flowing through the parasitic diode of the second switching element. Thus, efficiency during step-up operation (power generation periods) can be increased. Further, since the second switching element is turned on and off during regeneration periods, heat (loss) produced by current which flows through the second switching element in on periods can be reduced, whereby efficiency can be increased.
Preferably, a bootstrap circuit including a bootstrap capacitor is connected to a line extending between the battery and the drain of the first switching element, the bootstrap circuit being connected to a gate of the second switching element as a drive power source for the second switching element in order to apply electrical potential of the bootstrap capacitor to the gate of the second switching element, and the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns the first and second switching elements on and off alternately. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means turns the second switching element on and off and turns the first switching element on and off at a duty ratio in alternating periods of constant intervals.
In this case, the capacitor of the bootstrap circuit can be charged through the on-off operation of the first switching element during regeneration periods, whereby the second switching element can be turned on reliably during regeneration periods. Thus, it becomes possible to absorb regenerative current by the battery.
Preferably, a bootstrap circuit including a bootstrap capacitor may be connected to a line extending between the battery and the drain of the first switching element, the bootstrap circuit being connected to a gate of the second switching element as a drive power source for the second switching element in order to apply electrical potential of the bootstrap capacitor to the gate of the second switching element, and the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns the first and second switching elements on and off alternately. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means performs PWM control in order to turn only the second switching element on and off at a duty ratio, and restricts the duty ratio so as not to exceed a predetermined duty ratio.
In this case, during regeneration periods, the second switching element can be turned off without fail, so that, during such an off period, the electromagnetic energy accumulated in the coil can be absorbed by the battery. Further, at that time, the parasitic diode of the first switching element enters an on state, and the potential at the connection point between the bootstrap capacitor and the parasitic diode assumes the ground level, so that the bootstrap capacitor can be charged, and thus the second switching element can be driven.
Preferably, the motor-driven power steering apparatus further comprises load state judgment means for determining a load status of the motor, wherein the step-up circuit control means turns at least one of the first and second switching elements on and off on the basis of the steering state as detected by the steering state judgment means and the load state of the motor as determined by the load state judgment means.
In this case, since at least one of the first and second switching elements can be turned on and off on the basis of the steering state and the load state of the motor, proper control can be performed on the basis of the steering state and the load state.
Preferably, the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns only the first switching element on and off and maintains the second switching element off continuously if the load state judgment means determines that the motor is in a light load state, and turns the first and second switching elements on and off alternately if the load state judgment means determines that the motor is in a heavy load state. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means turns only the second switching element on and off.
Preferably, a bootstrap circuit including a bootstrap capacitor is connected to a line extending between the battery and the drain of the first switching element, the bootstrap circuit being connected a gate of the second switching element as a drive power source for the second switching element in order to apply electrical potential of the bootstrap capacitor to the gate of the second switching element, and the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns only the first switching element on and off and maintains the second switching element off continuously if the load state judgment means determines that the motor is in a light load state, and turns the first and second switching elements on and off alternately if the load state judgment means determines that the motor is in a heavy load state. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means turns the second switching element on and off and turns the first switching element on and off at a fixed duty ratio in alternating periods of constant intervals.
Alternatively, the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns only the first switching element on and off and maintains the second switching element off continuously if the load state judgment means determines that the motor is in a light load state, and turns the first and second switching elements on and off alternately if the load state judgment means determines that the motor is in a heavy load state. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means performs PWM control in order to turn only the second switching element on and off at a duty ratio and restricts the duty ratio so as not to exceed a predetermined duty ratio.
In these cases, when the motor is in a light load state, only the first switching element is turned on and off, and the second switching element is maintained off continuously, so that the heat generation and switching loss of the second switching element are reduced to zero, whereby efficiency can be improved.
Alternatively, a bootstrap circuit including a bootstrap capacitor is connected to the drain of the second switching element, the bootstrap circuit being connected a gate of the second switching element as a drive power source for the second switching element in order to apply electrical potential of the bootstrap capacitor to the gate of the second switching element, and the step-up circuit control means operates as follows. When the steering state judgment means determines that the motor is in a power generation state, the step-up circuit control means turns the first and second switching elements on and off alternately. When the steering state judgment means determines that the motor is in a regeneration state, the step-up circuit control means turns only the second switching element on and off.
In this case, during regenerative periods, the drain potential of the second switching element increases due to regenerative power even in a state in which the second switching element is maintained off continuously, so that the gate potential of the second switching element can be made higher than the source potential thereof. Therefore, the second switching element can be turned on in order to allow the battery to absorb regenerative current.
Preferably, the step-up circuit control means includes target output voltage setting means for setting a target output voltage of the step-up circuit; control computation means for performing at least computation for proportional control on the basis of the difference between the target output voltage and the detected output voltage and for outputting a computed value; and PWM computation means for performing PWM computation on the basis of the computed value so as to determine a duty ratio, wherein the first and second switching elements are turned on and off in accordance with the determined duty ratio; and the target output voltage setting means changes the target output voltage in accordance with an operation state parameter indicative of an operation state of the vehicle or the motor.
In this case, since the target output voltage setting means changes the target output voltage in accordance with the operation state parameter indicative of an operation state of the vehicle or the motor, the first and second switching elements can be turned on and off in accordance with the operation state.
Preferably, the step-up circuit control means includes target output voltage setting means for setting a target output voltage of the step-up circuit; control computation means for performing at least computation for proportional control on the basis of the difference between the target output voltage and the detected output voltage and for outputting a computed value; and PWM computation means for performing PWM computation on the basis of the computed value so as to determine a duty ratio, wherein the first and second switching elements are turned on and off in accordance with the determined duty ratio; and the step-up circuit control means restricts the duty ratio so as not to exceed a predetermined duty ratio.
In this case, since duty ratio restriction is provided, breakage of the step-up circuit can be prevented in both power generation and regeneration periods.
Preferably, the motor-driven power steering apparatus further comprises status parameter detection means for detecting a parameter indicative of states of the step-up circuit; and judgment means for comparing the parameter detected by the status parameter detection means with a judgment value and judging whether or not the step-up circuit is normal, wherein the step-up circuit control means stops step-up control for the step-up circuit when the judgment means judges that the step-up circuit is not normal.
In this case, when the step-up circuit suffers a failure, the step-up control for the step-up circuit can be stopped, whereby breakage of the step-up circuit in an anomalous state can be prevented.
Preferably, the motor-driven power steering apparatus further comprises first open-close means connected to a battery-voltage supply section of the step-up circuit and turned on and off by the step-up circuit control means; and second open-close means for controlling supply of electrical power to the motor, wherein when the judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means turns the first and second open-close means off.
In this case, when the step-up circuit suffers a failure, the motor-driven power steering apparatus can be switched to a manual steering mode. In addition, since no regenerative current flows into the step-up circuit even when the motor enters a regeneration state, breakage of circuit elements of the step-up circuit can be prevented.
Preferably, when the judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means maintains the first switching element off continuously and maintains the second switching element on continuously.
In this case, when the step-up circuit suffers a failure, assist control at the battery voltage can be continued. In addition, during regeneration periods, regenerative current can be absorbed by the battery.
Preferably, the motor-driven power steering apparatus further comprises first open-close means connected to a battery-voltage supply section of the step-up circuit and turned on and off by the step-up circuit control means; and a circuit which includes a first resistor connected to a connection point between the drain of the first switching element and the battery-voltage supply section and which receives an ignition voltage when an ignition switch is turned on. The step-up circuit control means includes first element control means, operated when the ignition switch is turned on, for turning at least the first switching element on or off before the first open-close means is turned on; drain voltage detection means for detecting drain voltage of the first or second switching element; and first failure judgment means for comparing the drain voltage with a first failure judgment value and determining whether the step-up circuit suffers a failure.
In this case, the judgment as to whether the step-up circuit suffers a failure can be performed in an initial check stage after the ignition switch is turned on.
Preferably, the motor-driven power steering apparatus further comprises second open-close means for controlling supply of electrical power to the motor, wherein when the first failure judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means turns the first and second open-close means off.
In this case, when the step-up circuit is judged to suffer a failure in an initial check stage after the ignition switch is turned on, the failsafe mechanism operates.
Preferably, the motor-driven power steering apparatus further comprises second open-close means for controlling supply of electrical power to the motor, wherein when the first failure judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means turns the first and second open-close means on, maintains the first switching element off continuously, and maintains the second switching element on continuously.
In this case, even when the step-up circuit is judged to suffer a failure in an initial check stage after the ignition switch is turned on, assist control at the battery voltage can be performed, although assist control at boosted voltage produced by means of the step-up circuit cannot be performed. In addition, regenerative current can be absorbed by the battery during regeneration periods.
Preferably, the motor-driven power steering apparatus further comprises first open-close means connected to a battery-voltage supply section of the step-up circuit and turned on and off by the step-up circuit control means; and a circuit which includes a second resistor connected to the drain of the second switching element and which receives an ignition voltage when an ignition switch is turned on. The step-up circuit control means includes second element control means, operated when the ignition switch is turned on, for turning the first and second switching elements on simultaneously, turning the first and second switching elements off simultaneously, or turning the first switching element off and the second switching element on, before the first open-close means is turned on; drain voltage detection means for detecting drain voltage of at least the first switching element; and second failure judgment means for comparing the drain voltage with a second failure judgment value and determining whether the step-up circuit suffers a failure.
In this case, the judgment as to whether the step-up circuit suffers a failure can be performed in an initial check stage after the ignition switch is turned on.
Preferably, the motor-driven power steering apparatus further comprises second open-close means for controlling supply of electrical power to the motor, wherein when the second failure judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means turns the first and second open-close means off.
In this case, when the step-up circuit is judged to suffer a failure in an initial check stage after the ignition switch is turned on, the failsafe mechanism operates.
Preferably, the motor-driven power steering apparatus further comprises second open-close means for controlling supply of electrical power to the motor, wherein when the second failure judgment means judges that the step-up circuit suffers a failure, the step-up circuit control means turns the first and second open-close means on, maintains the first switching element off continuously, and maintains the second switching element on continuously.
In this case, even when the step-up circuit is judged to suffer a failure in an initial check stage after the ignition switch is turned on, assist control at the battery voltage can be performed, although assist control at boosted voltage produced by means of the step-up circuit cannot be performed. In addition, regenerative current can be absorbed by the battery during regeneration periods.